1. Field of the Invention
The invention relates to a micro pressure sensor, and more particularly to a micromachined capacitive pressure sensor with a two-level suspended structure, wherein the micro pressure sensor is applied to the fluid pressure detection, the tactile pressure detection, and particularly the fingerprint pattern detection. Detailed descriptions regarding the capacitive pressure fingerprint sensor may be found in a commonly-owned, co-pending U.S. patent application Ser. No. 10/434,833, filed May 13, 2003 and entitled “PRESSURE TYPE FINGERPRINT SENSOR FABRICATION METHOD,” and a commonly-owned, co-pending Taiwan Patent Application No. 090112023, filed May 17, 2001 and entitled “CAPACITIVE MICRO PRESSURE SENSING MEMBER, METHOD FOR MANUFACTURING THE SAME, AND METHOD FOR READING SIGNALS THEREOF.”
2. Description of the Related Art
There are many methods for manufacturing capacitive pressure sensors using the micro-electro-mechanical system (MEMS) technology including the bulk micromachining and the surface micromachining. However, the structure of the sensor never departs from a capacitor composed of a pair of parallel plates, and a reference pressure chamber sealed between the parallel plates, as shown in FIG. 1, which is briefly a structural cross-sectional view showing a typical capacitive micro pressure sensing member 10. The capacitive micro pressure sensing member 10 includes a variable capacitor, which is composed of a pair of parallel plates and comprises a floating electrode 12 and a fixed electrode (not shown) located above a substrate 14. A sealed chamber 16 having a reference pressure P0 is formed between the floating electrode 12 and the substrate 14. When the external pressure P1 is different from the pressure P0 in the sealed chamber 16, as shown in FIG. 2, a force due to the pressure difference is induced to the floating electrode 12 and cause it to deform. Since the boundary of the floating electrode 12 is connected and fixed to the substrate 14, the maximum deformation d occurs at the center of the floating electrode 12, the zero deformation occurs at the boundary of the floating electrode, and the magnitude of deformation decreases from the center to the fixed boundary of the floating electrode. The critical feature of the structure is that the capacitance variation owing to the pressure difference of this conventional parallel-plate capacitive micro pressure sensing member is limited to the nonlinear deformation of the floating electrode plate, and the sensitivity of the sensing member 10 is also limited accordingly.
Since the capacitance is directly proportional to the overlapped area of the pair of parallel plates and inversely proportional to the gap between the two parallel plates, one typical method is to enlarge the area of the floating electrode 12 to solve the problem. However, this method increases the chip cost. Alternatively, another typical method is to reduce the gap length of the parallel-plate capacitor using the surface micromachining technology. However, this further causes another problem of film deformation under residual stresses or another problem of sticking the diaphragm structure onto the substrate.
Consequently, the invention solves the above-mentioned problems by providing a novel micro pressure sensing member having a two-level suspended structure including an electrode plate having a maximum translation displacement to generate a maximum sense capacitance variation and enhance the sensitivity when the external pressure changes. The capacitive micro pressure sensing members also may be formed into an array, which may be applied to a fingerprint sensor and precisely recognize the ridge shape of the fingerprint.